Process of making flexible displays comprising wax-encapsulated bichromal spheres

ABSTRACT

A display comprised of a first component containing spheres encapsulated within a wax, and thereover and thereunder said component substrates.

This application is a divisional of Application No. 09/814,222 filedMar. 21, 2001, now U.S. Pat. No. 6,419,982 which is a Divisional of Ser.No. 09/035,518, filed Mar. 5, 1998 now U.S. Pat. No. 6,235,395 issuedMay 22, 2001.

PENDING APPLICATION

Illustrated in copending application, U.S. Ser. No. 09/035,590, filedMar. 5, 1998, now U.S. Pat. No. 5,989,629, issued Nov. 23, 1999, thedisclosure of which is totally incorporated herein by reference, andwhich application is being filed concurrently herewith is a process forthe preparation of bichromal spheres comprising (I) preparingmonochromal spheres by the aggregation and coalescence of an emulsionresin with a first colorant and an inorganic salt; (ii) contacting theresulting monochromal spheres with an oxidizing agent, followed by apolymerization with a vinyl monomer and a free radical initiator; (iii)forming a of the resulting monochromal spheres on a substrate; and (iv)subjecting the resulting monochromal spheres to a vapor thermaldeposition with a second colorant dissimilar than the first colorant tothereby coat one hemisphere of each of said monochromal spheres andresulting in bichromal spheres with dissimilar colors.

BACKGROUND OF THE INVENTION

This invention generally relates to displays, and the preparation, orfabrication of a display devices, and more specifically to robust andreflective flexible display devices, and sheets thereof comprised ofsmall spheres encapsulated with a wax, for example, wherein small is forexample, from about 2 to about 150 microns in volume average diameter asmeasured by the Coulter Counter, and more specifically wherein the waxencapsulated sphere is comprised of a bichromal sphere encapsulatedwithin a hydrocarbon wax, and wherein the wax encapsulated spheres aredispersed in an elastomer or plastic membrane with a plastic conductivecoating such as indium tin oxide.

The displays are useful in generating images, which can be stored orerased and more specifically the display devices or devices function byrotating a bichromal sphere by an external field to create the image.

PRIOR ART

Electric Paper or twisted ball panel display devices are known and aredescribed, for example, in U.S. Pat. Nos. 4,126,854; 4,143,103;4,261,653; 4,438,160; 5,389,945, the disclosures of each of which aretotally incorporated herein by reference, and wherein the devices aregenerally comprised of an elastomer, such as a cured polysiloxane,sandwiched between two ITO coated substrates, such as glass or MYLAR™,and wherein the elastomer layer has closely packed cavities containing abichromal sphere suspended in a dielectric liquid. The image is formedby the application of an electric field, which rotates the bichromalsphere differentiated by the different static properties induced by thepigment contained in the sphere and resulting in contrasting zetapotentials of the two colored hemispheres of the sphere. It is thedifference in zeta potentials between the hemispheres of the ball whichcauses the ball to act like a dipole in the presence of an electricalfield, causing the ball to rotate, until its dipole vector lines up withthe direction of the electrical field established between opposedelectrodes. In addition to the dipole charge distribution found on thebichromal ball in the presence of an electric field, there is also amonopole charge which is the net electrical charge of the entire ball.As a result of the monopole charge, the ball is caused to move in thedirection of the electrical field and will rest and be retained againsta cavity wall. In order for the ball to rotate easily in the liquidwithin the cavity, due to the dipole charge, it is moved from contactwith the cavity wall. When at rest against the cavity wall, friction andother forces will prevent it from rotating until it has been moved awayagain, due to the monopole charge. It is this feature which primarilyenables long term, that is indefinite image retention in the displaydevice if undisturbed or not handled by force. However, theaforementioned electric paper devices are not believed to be robust andsuffer with respect to image retention when handled. More specifically,the image formed on the above prior art displays can deteriorate bytouching it, especially if the user has built up a static charge, byrubbing over the image, dropping the display device on a surface or bycarrying it. This loss in image retention is caused by the insufficientlack of adhesion of the bichromal sphere onto the elastomer, and whereinthe spheres detachment from the cavity surface suspends the bichromalsphere in a low viscosity medium thus allowing the sphere to rotateuncontrollably within the liquid cavity. The robust flexible displaydevice of the present invention, possesses minimum, or substantially noimage deterioration when handled by the user. More specifically, thedisplay device of the present invention contains a solid waxencapsulating the bichromal sphere, hence not allowing the for thesphere to move or minimizing movement within the cavity.

The fabrication of certain bichromal spheres is known, for example, theabove mentioned U.S. Pat. No. 4,143,103, and wherein the sphere iscomprised of black polyethylene with a light reflective material, forexample, titanium oxide, sputtered on hemisphere. Also in U.S. Pat. No.4,438,160, the disclosures of which is totally incorporated herein byreference, a rotary ball is prepared by coating white glass balls ofabout 50 microns in diameter, with an inorganic coloring layer such asMgF₂ or Sb₂S₃ by evaporation. In a similar process, there is disclosedin an article entitled “The Gyricon—A twisting Ball Display”, publishedin the proceedings of the S.I.D., Vol. 18/3 and 4 (1977), a method forfabricating bichromal balls by first heavily loading chromatic glassballs with a white pigment such as titanium oxide, followed by coatingfrom one direction in a vacuum evaporation chamber with a dense layer ofnonconductive black material which coats only one hemisphere.

Also in U.S. Pat. No. 4,810,431 by Leidner, there is disclosed a processfor generating spherical particles by (a) coextruding a fiber of asemi-circular layer of a polyethylene pigmented white and a black layerof polyethylene containing magnetite, (b) chopping the resultant fiberinto fine particles ranging from 10 microns to about 10 millimeters, (c)mixing the particles with clay or anti-agglomeration materials, and (d)heating the mixture with a liquid at about 120° C. to spherodize theparticles, followed by cooling to allow for solidification.

There is also disclosed in U.S. Pat. No. 5,262,809, an apparatus forfabricating hemispherical bichromal balls, comprising a separator memberhaving opposing first and second surfaces and an edge region in contactwith both surfaces, and delivery means for flowing first and secondcolored hardenable liquid material over the first and second surfaces,respectively, so that the liquid materials arrive at the edge atsubstantially the same flow rate and form a reservoir outboard of theedge region. The reservoir comprises side-by-side regions of differentcolors which do not intermix. Further means is provided for propellingthe first and second liquid materials away from the separator member andout of the reservoir into a fluid medium as a plurality of side-by-sidebichromal streams whose forward ends become unstable and break up intodroplets which form into spherical balls, each of the balls comprisinghemispheres of differently colored hardenable liquids. These bichromalballs are from about 5 to 200 microns in diameter.

There is a need for a robust and reflective flexible display deviceswhich an reimageable multiple times, such as from about 10 to about10,000 times and preferably from about 100 to about 10,000 times, whichdisplays high reflectivity, such as from about 15 to about 100 percentor preferably from about 20 to about 50 percent, which displays highcontrast ratios such as from about 3 to about 6, and is robust such thatthe device can be handled like paper without image deterioration orimage loss.

SUMMARY OF THE INVENTION

It is an feature of the present invention to provide a method for thepreparation of a robust and flexible electric papers, gyricon ortwisting Ball display devices

In another feature of the present invention there is provided a processfor the fabrication of wax encapsulated bichromal spheres.

In yet another feature of the present invention there are providedprocesses for the preparation of a robust device comprised of waxencapsulated bichromal spheres, with one hemisphere displaying a whitecolor, and the other hemisphere displaying a black color, and whereineach of the bichromal spheres is each of from about 2 to about 50microns in diameter, and preferably of from about 5 to about 25 micronsin diameter, and which spheres are for example, dispersed in anelastomer or plastic coated with a conductive coating like indium tinoxide.

Moreover, it is an feature of the present invention to provide processesfor the preparation of wax encapsulated bichromal spheres bycoacervation involving the precipitation of wax onto the surface of eachbichromal sphere.

In yet another feature of the present invention there is provided aprocess for uniformly coating bichromal spheres with a wax component.

A further feature of the present invention is to provide a robustflexible display device with high reflectivity, such as from about 15 toabout 100 percent, and preferably higher than 18 percent, for examplefrom about 20 to about 75 percent.

Moreover, a further feature of the present invention is to provide arobust flexible display device with high contrast ratio, such as fromabout 3 to about 9.

Additionally, a further feature of the present invention is to provide arobust flexible display device which can be handled like paper withoutimage deterioration or loss.

The bichromal spheres of the present invention which are comprised forexample, of hemispheres of contrasting, or different color anddissimilar zeta potentials are useful as a flexible “electric paper”display device. The present invention is generally directed to a robustelectric paper display device, or a rotary twisted ball or a Gyricondisplay device, comprised of bichromal sphere such as from about 45 toabout 65, and more specifically from about 45 to about 55 percent byweight of the device, in which one hemispherical, that is about onehalf, or about 50 percent, surface of the sphere ball is colored a firstcolor, like white and the other hemisphere is of a second dissimilarcolor, that is for example, a color other than white, such as black,reference for example U.S. Pat. No. 4,126,854, the disclosure of whichis totally incorporated hereinby reference. The wax encapsulant is forexample a low melting hydrocarbon wax, for example with a melting pointof from about 10 degrees Centigrade to about 80, and more specificallyform about 25 to about 65 degrees Centigrade, and wherein the carbonchain length or carbon content of the wax is from about 18 to about1,000, and more specifically from about 100 to about 500 carbon atoms.The flexible display device, when heated to a temperature above themelting point of the wax, and under the action of an external electricfield, allows the bichromal spheres to rotate in accordance with theirelectrical anisotropy to provide an image. Cooling the device to atemperature at or below about the melting point of the wax, freezes orimmobilizes the image.

The present invention is also directed to a method for the preparationof display devices and methods for fabricating the wax encapsulatedbichromal spheres, and wherein each of the spheres are of small size,such as from about 2 to about 150 microns in diameter and preferablyfrom about 10 to about 50 microns in diameter, volume averagethroughout, and wherein the method comprises (a) solubilizing a wax inan organic solvent; (b) suspending bichromal spheres in the organicsolvent; (c) adding thereto a second solvent which precipitates orcoacervates the wax onto each of the bichromal spheres; and (d)optionally, but preferably isolating the wax encapsulated spheres.

The bichromal spheres are also preferably fabricated by the processes asdisclosed in copending application Ser. No. 09/035,590, filed Mar. 5,1998, the disclosure of which are totally incorporated herein byreference, or as disclosed in U.S. Pat. No. 4,126,854, the disclosure ofwhich are totally incorporated herein by reference. Alternatively, otherknown methods for the preparation of bichromal spheres can be practicedsuch as disclosed in U.S. Pat. No. 5,262,809, the disclosure of which istotally incorporated herein by reference.

With the present invention, there is provided a robust flexible displaydevice, and wherein the device is comprised of wax encapsulatedbichromal spheres dispersed in an elastomer or transparent plasticdevice with a plastic conductive coating. An image can be formed byheating the display device, or display sheet in for example, anionographic apparatus, to a temperature of about 10 to about 20 degreesCentigrade above the melting point of the wax, causing the wax to meltinto a low viscosity liquid, such as a viscosity of from about 1 toabout 100 centipoise, followed by forming an image with an externalelectric field or ion deposited on the display device and casing thebichromal spheres to rotate in the liquid wax cavity, and followed byremoving the image from the heated device and allowing the wax tosolidify on cooling to about the melting point of the wax or lower andfreezing or immobilizing the bichromal sphere to result in a robustflexible device.

The wax encapsulated spheres can be prepared by coating the bichromalspheres with wax as practiced in the art of coacervation. In anembodiment of the present invention, the wax encapsulated spheres areprepared by a precipitation technique, for example, by suspending thebichromal spheres in an organic solvent containing a dissolved waxcomponent, followed by adding thereto a solvent within the wax componentis insoluble, or substantially insoluble thereby causing it toprecipitate from solution, and wherein, the wax precipitates and coatsonto the bichromal spheres thereby encapsulating it.

The process of the present invention in embodiments comprises theencapsulation of bichromal spheres with a wax components, followed bydispersing the wax encapsulated bichromal spheres in an elastomer orplastic device situated on a substrate, such as MYLAR™ coated withindium tin oxide. For example, about 20 to about 25 grams of 70 micronbichromal spheres prepared as disclosed in U.S. Pat. No. 5,262,809, thedisclosure of which is totally incorporated herein by reference, issuspended in about 150 to about 200 grams of tetrahydrofuran containing15 to about 20 grams of Petrolite X-6040 wax (mp=35° C.). To thisstirred mixture is then added about 100 to about 150 grams of methanoldropwise over a 1 hour period, causing the wax to precipitate and coatthe bichromal spheres. The product is then filtered through a 50 micronscreen to result in about 50 to about 55 grams of wax encapsulatedbichromal sphere.

Illustrative examples of wax components that can be utilized forencapsulating the bichromal spheres are known, for example linear andbranched hydrocarbons of from about 18 to about 1,000, and morespecifically from about 25 to about 100 carbon atoms, and with a meltingpoint temperature of from about 15° C. to about 80° C., and preferablyfrom about 25° C. to about 60° C., and with molecular weights of fromabout 300 grams per mole to about 10,000 grams per more as measured byGel Permeation Chromatography Preferably. Examples of commercial waxesare the Petrolite X-series wax, or bees wax. Other useful encapsulatesare low melting hydrocarbons such as Norpar, Illustrative examples oforganic solvents that can be utilized for dissolving the wax componentinclude nonpopular solvents such as hexanes, heptanes, octanes, Isopar,Magisol, ether, tetrahydrofuran, toluene, xylene, ethyl acetate,benzene, naphtha and the like. Generally organic aromatic solvents andaliphatic solvents can be selected. Illustrative examples of solventsthat can be utilized for precipitating or coacervating the wax onto thesurface of the bichromal spheres include any solvent for this purpose,such as water, acetone, and aliphatic alcohols, with for example, fromabout 1 to about 25 carbon atoms, such as methanol, ethanol, propanol,and butanol.

Illustrative examples of the first substrate utilized for forming thedevices are an epoxy resin, an elastomer such as polydimethyl siloxane,elastomers such as SYLGARD 184 available from Dow Corning, apolyurethane, a polyurea, a polyester, and the like. Examples of thesecond and third substrate are glass, a polyester such as MYLARTM, apolyurethane, a polystyene resin, or a polystyrene-(meth)-acrylate resinwith a conductive layer thereon such as an indium tin oxide coating, andwherein the thickness of each of the first, second, and third substratesare from about 20 to about 1,000 microns and preferably from about 30 toabout 500 micron, and the thickness of the conductive coating is fromabout 0.5 micron to about 20 micron.

Aspects of the present invention relate to

a display comprised of a first component containing spheres encapsulatedwithin a wax, and thereover and thereunder the first componentsubstrates;

a display device comprised of a first substrate containing an array ofbichromal spheres encapsulated with a wax, and wherein the firstsubstrate is situated between a second substrate and a third substrate;

a display device wherein the first substrate is an elastomer or aplastic;

a display device wherein the second substrate is an indium tin oxidecoated glass or a polyester of polyethylene-terephthalate;

a display wherein the second substrate is indinium tin oxide;

a display device wherein the bichromal sphere is from about 5 to about25 microns in diameter, and is comprised of one hemisphere coloredwhite, and the remaining hemisphere coated with a color other thanwhite;

a display wherein the other that white is black, blue, red, yellow,cyan, green, magenta, orange, green, or mixtures thereof;

a display device wherein the bichromal sphere is encapsulated in a waxcomprised of a hydrocarbon.

a display device wherein the wax is a transparent linear hydrocarbon ora branched hydrocarbon each with from about 18 to about 1,000 carbonatoms;

a display device wherein the first substrate is an elastomer ofpolydimethylsiloxane or a polyurethane;

a display device wherein the first substrate is a plastic of an epoxyresin, a polyester resin, a polyamide resin, apolystyrene-(meth)-acrylate resin, a polydimthylsiloxane, or apolyurethane;

a display device wherein the second and third substrate is a conductiveglass;

a display device wherein the second substrate is a conductive plastic ofa polyester resin coated with indinium tin oxide;

a process for the preparation of wax encapsulated bichromal spherescomprising (a) solubilizing a wax in an organic solvent; (b) suspendingbichromal spheres in the organic solvent; (c) adding thereto a secondsolvent which precipitates or coacervates the wax onto each of thebichromal spheres; and (d) optionally isolating the wax encapsulatedspheres;

a process wherein the wax is a linear or branched hydrocarbon with about18 to about 1,000 carbon atoms;

a process wherein the wax has a melting point of from about 15 degreesCentigrade to about 80 degrees Centigrade;

a process wherein the organic solvent is hexane, heptane, octane,nonane, decane, dodecane, Isopar, tetrahydrofuran, diethyl ether,chloroform, methylene chloride, ethyl acetate, benzene, chlorobenzene,toluene, n-methylpyrrolidinone, dimethyl sulfoxide, dimethyl formamideor xylene;

a process wherein the second solvent is water, methanol, ethanol,propanol, butanol, or acetone;

a display device comprised of a first substrate with a thickness of fromabout 20 to about 100 microns, containing wax encapsulated bichromalspheres of from about 45 to about 85 percent by weight of the substrate,wherein the first substrate is situated between a second and thirdsubstrate;

a display device comprised of a first substrate comprised of a resin orelastomer and dispersed therein an array of bichromal spheresencapsulated with a wax, and wherein the first substrate with saidspheres has a thickness of from about 20 to about 100 microns, and issandwiched between a second and third substrate comprised of aconductive glass or plastic device each with a thickness of from about10 to about 500 microns in diameter;

a device wherein said second and said third substrate are comprised of aconductive glass or plastic device;

a process wherein the organic solvent is hexane, heptane, octane,nonane, decane, dodecane, Isopar, tetrahydrofuran, diethyl ether,chloroform, methylene chloride, ethyl acetate, benzene, chlorobenzene,toluene, n-methylpyrrolidinone, dimethyl sulfoxide, dimethyl formamideor xylene, and which solvent is selected in an amount of from about 5 toabout 40 percent by weight of the bichromal spheres; and

a process wherein the second solvent is water, methanol, ethanol,propanol, butanol, or acetone, and which solvent is selected in anamount of from about 5 to about 40 percent by weight of the bichromalspheres.

The following EXAMPLES are provided:

EXAMPLE 1

Preparation of a robust and reflective display device comprised of 70volume average micron bichromal spheres encapsulated within 25% byweight of hydrocarbon wax (Petrolite X-6040), dispersed in a siloxaneelastomer and sandwiched between two indinium tin oxide-(ITO) coatedglass or MYLAR™ substrates was accomplished as follows:

Step 1. Preparation of 70 Micron Bichromal Spheres Encapsulated With 25%By Weight of Wax (Petrolite X-6040):

A 1 liter beaker equipped with a mechanical stirrer was charged with 220grams of tetrahydrofuran solvent, 75 grams of 70 micron black and whitecolored bichromal spheres (prepared as disclosed in U.S. Pat. No.5,262,809, the disclosure of which is totally incorporated herein byreference) and 18.75 grams of Petrolite X-6040 wax (mp=35° C.). To thisstirred mixture was then added about 125 grams of methanol dropwise overa 3 hour period, causing the wax to precipitate and fully coat each ofthe bichromal spheres. The product was then filtered through a 50 micronscreen to result in about 85 grams of wax encapsulated black and whitecolored bichromal spheres.

Step 2. Preparation of the Display Device

A display device was fabricated from the above prepared spheres by (1)mixing 50 grams of the prepared bichromal spheres with 50 grams ofSYIGARD^(R) 185 silicone elastomer Kit available from Dow Corning; (2)forming a device by spreading the resulting mixture on a glass platesurface and which spreading was with a metering bar such as an 8-PathWet Film Applicator (available from P. Gardner Company) with a gap offrom about 20 microns to about 500 microns, and crosslinking the SYLGARDelastomer device by heating to a temperature of from about 80° C. toabout 100° C. for a duration of from about 3 to about 24 hours; and then(3) sealing the device between addressing plates of indium tin oxidecoated glass or preferably MYLAR™, each with a thickness of from about20 to about 500 microns.

EXAMPLE 2

Preparation of a robust and reflective display device comprised of 70micron Bichromal spheres encapsulated within 20% by weight ofhydrocarbon wax (Petrolite X-6040), dispersed in a siloxane elastomerand sandwiched between two MYLAR™ substrates was accomplished as follows

Step 1. Preparation of 70 Micron Bichromal Spheres Encapsulated with 20Percent By Weight of Wax (Petrolite X-6040):

A 1 liter beaker equipped with a mechanical stirrer was charged with 220grams of tetrahydrofuran solvent, 75 grams of 70 micron black and whitecolored bichromal spheres (prepared as disclosed in U.S. Pat. No.5,262,809) and 15 grams of Petrolite X-6040 wax (mp=35° C.). To thisstirred mixture was then added about 125 grams of methanol dropwise overa 3 hour period, causing the wax to precipitate and fully coat thebichromal spheres. The product was then filtered through a 50 micronscreen to result in about 83 grams of wax encapsulated bichromalspheres.

Step 2. Preparation of the Display Device

A display device was fabricated from the above bichromal spheres by (1)mixing 50 grams of the prepared bichromal spheres with 50 grams ofSYIGARD^(R) 185 silicone elastomer Kit available from Dow Corning; (2)forming a device by spreading the mixture on a glass plate surface andwhich spreading was with a metering bar such as an 8-Path Wet FilmApplicator (available from P. Gardner Company) with a gap of from about20 microns to about 500 microns, and crosslinking the SYLGARD elastomerdevice by heating to a temperature of from about 80° C. to about 100° C.for a duration of from about 3 to about 24 hours; and (3) then sealingthe device between addressing plates of indium tin oxide, coated glassor MYLAR™, each with a thickness of from about 20 to about 500 micronsin diameter.

EXAMPLE 3

Preparation of a robust and reflective display device comprised of 70micron Bichromal spheres encapsulated within 25% by weight ofhydrocarbon wax (Petrolite), dispersed in a siloxane elastomer andsandwiched between two substrates was accomplished as follows

Step 1. Preparation of 70 Micron Bichromal Spheres Encapsulated with 25%By Weight of Wax (Petrolite X-6028):

A 1 liter beaker equipped with a mechanical stirrer was charged with 220grams of tetrahydrofuran solvent, 25 grams of 70 micron of black andwhite colored bichromal spheres (prepared as disclosed in U.S. Pat. No.5,262,809) and 18 grams of Petrolite(X-6028) wax (mp=41° C.). To thisstirred mixture was then added about 125 grams of methanol dropwise overa 3 hour period, causing the wax to precipitate and fully coat each ofthe bichromal spheres. The product was then filtered through a 50 micronscreen to result in about 85 grams of wax encapsulated bichromalspheres.

Step 2. Preparation of the Display Devices.

A display device was fabricated from the above bichromal sphere by (1)mixing 50 grams of the prepared bichromal spheres with 50 grams ofSYIGARD^(R) 185 silicone elastomer Kit available from Dow Corning; (2)forming a device by spreading the mixture on a glass plate surface andwhich spreading was with a metering bar such as an 8-Path Wet FilmApplicator (available from P. Gardner Company) with a gap of from about20 microns to about 500 microns, and crosslinking the SYLGARD elastomerdevice by heating to a temperature of from about 80° C. to about 100° C.for a duration of from about 3 to about 24 hours; and (3) then sealingdevice between addressing plates of Indium tin oxide coated glass orMYLAR™, each with a thickness of from about 20 to about 500 microns indiameter.

Other modifications of the present invention may occur to one ofordinary skill in the art subsequent to a review of the presentapplication, and these modifications, including equivalents, orsubstantial equivalents thereof, are intended to be included within thescope of the present invention.

What is claimed is:
 1. A process for the preparation of the waxencapsulated bichromal spheres consisting essentially of (a)solubilizing a wax in an organic solvent; (b) suspending bichromalspheres in said organic solvent; (c) adding thereto a second solventwhich precipitates or coacervates the wax onto each of said bichromalspheres; and (d) optionally isolating the wax encapsulated spheres.
 2. Aprocess in accordance with claim 1 wherein the wax is a hydrocarbon wax.3. A process in accordance with claim 1 wherein the wax is a bees-wax.4. A process in accordance with claim 1 wherein the organic solvent isselected in an amount of from about 5 to about 40 percent by weight ofthe wax encapsulated spheres.
 5. A process in accordance with claim 1wherein the second solvent is selected in an amount of from about 5 toabout 40 percent by weight of the wax encapsulated spheres.
 6. A processin accordance with claim 1, wherein the wax encapsulated spherecomprises two hemispheres of dissimilar color and said spheres are fromabout 2 to about 150 microns in diameter.
 7. A process in accordancewith claim 1, wherein the wax encapsulated spheres comprises twohemispheres of dissimilar color and said spheres are from about 5 toabout 25 microns in diameter.
 8. A process in accordance with claim 1,wherein one of the colors of the two hemispheres of dissimilar colors iswhite and the other color is black.